1. Field of the Invention
This invention relates to semiconductor analog-to-digital converters and more particularly, but not by way of limitation, to a switch for use in an analog-to-digital converter.
2. Description of the Prior Art
Referring to FIG. 1, a typical potentiometric analog-to-digital converter can be observed. As can be seen, a large number of resistors and analog switches is required to handle a small number of bits. In the industry, much effort has been made to reduce the number of required resistors and switches.
As can be seen, the industry's standard is now a monolithic successive approximation analog-to-digital converter. This converter uses a sampled-data comparator and a multi-tapped diffused-resistor string (ladder) between the reference voltage (V.sub.ref) and ground. A tap is provided for each of the 2.sup.N possible analog voltage levels. Note that for an eight bit design, 256 resistors and 510 (2.sup.N+1 -2) analog decoder switches are needed. Eight switches are placed in a series between each tap on the ladder and the comparator input. By a successive approximation technique, a tree decoder differentially arrives at the voltage level approximating the analog input.
Referring now to FIG. 1, there is shown an analog level output line 10, a resistor ladder network 12, a comparator 14, an analog input 16, digital inputs 18 and switches or tree decoder 20. Further, note that the resistor ladder network 12 is connected in series and supplied with a voltage reference at one end and grounded at the other end. Locations in the resistor ladder network 12 are addressed by the digital inputs 18 and read out through the output line 10. This output is compared to the analog input 16 by the comparator 14 which addressed control logic to determine the next resistor ladder network location. By successive approximation, a level nearest the analog input 16 is obtained.